Climate MattersJune 25, 2024

Scorching Days Followed by Sweltering Nights


  • Extremely hot summer days are dangerous. And when they’re followed by extremely hot nights, the health risks grow.

  • Sweltering summer nights limit our ability to cool off from scorching summer days, prolonging heat stress and amplifying health risks. 

  • The frequency of these risky combined hot summer day-night events has increased in 202 U.S. cities (84% of the 241 analyzed by Climate Central) since the 1970s. 

  • Studies have shown that this trend extends across the Northern Hemisphere, and is mainly driven by human-caused carbon pollution. 

  • Climate Central’s Climate Shift Index quantifies the influence of climate change on daytime highs and nighttime lows across the globe, every day. 

Download local data

Scorching days followed by sweltering nights

As our climate warms, summer is heating up across the U.S. But summer warming isn’t all about scorching days. It’s also about sweltering nights. 

Summer temperatures are also rising after the sun goes down. In fact, summer nighttime lows across the U.S. have warmed nearly twice as fast as summer daytime highs since records began in 1895. 

Since 1970, summer nights have warmed in 230 U.S. locations analyzed by Climate Central — by 3°F on average.

When nights don’t cool off enough relative to peak daytime temperatures, people have a harder time cooling off and recovering from the heat of the day.  

Heat is the deadliest weather-related hazard in the U.S. And reduced sleep during sweltering nights can prolong heat stress and related health risks during the hottest time of year. 

Compared to individual hot days or nights, the combination of both hot days and hot nights are even more risky for human health. New analysis shows that these combined hot summer days and nights are on the rise across the U.S.

CM: Combined Hot Summer Days and Nights 1970 to 2023 (EN)
Click the downloadable graphic: Combined Hot Summer Days and Nights 1970 to 2023

More combined hot summer days and nights

To understand how 24-hour summer heat extremes are changing as the season warms, Climate Central analyzed the annual frequency of hot summer days followed by hot summer nights (above the daily 90th percentile) from 1970 to 2023 in 241 U.S. cities (see Methodology below for details). 

Of the 241 cities analyzed, 202 (84%) now experience more combined hot summer days and nights than in the 1970s. 

These 202 cities experienced an average of five more combined hot summer day-night events each summer during the last decade (2014-2023) than during the 1970s. 

This rising trend was observed across the entire country, and was strongest across the western and southern U.S., led by: Monterey, Calif. (which now experiences an average of 18 more events each summer); Bend, Ore. (16 more events); and Austin, Texas (16 more events). 

CM: More Combined Hot Summer Days and Nights 2024 (EN)
Click the downloadable graphic: More Combined Hot Summer Days and Nights

Climate change is causing more hot summer days and nights

Scorching days followed by sweltering nights limit the body’s ability to efficiently cool off without costly and energy-intensive air conditioning. 

And as the planet heats up, these dangerous combined hot summer days and nights are on the rise in the U.S. and across the world.

According to a 2020 study, extremely hot summer days followed by extremely hot nights have become significantly more frequent and intense across the Northern Hemisphere from 1960 to 2012. 

The same study found that human-caused carbon pollution was the leading cause of this dangerous trend.  If moderate to very high levels of carbon pollution continue, co-occurring extremely hot summer days and nights are likely to happen four to eight times more often by 2100 (relative to 2012 levels).

According to Sleepless Nights, a new Climate Central report, human-caused climate change accounts for the rise in hot summer nights (at least 77°F) across the globe from 2014 to 2023. In the U.S., the rise in hot summer nights due to climate change has been strongest in Florida, Texas, Louisiana, Nevada, and Arizona.

To bring these long-term trends down to the present day, Climate Central’s Climate Shift Index map tool quantifies the influence of carbon pollution on daily high and low temperatures across the U.S. and around the world. 

Risks and unequal burdens of nighttime heat

Sweltering nights limit our ability to cool off and recover from extremely hot summer days. This can lead to greater heat stress and related health risks—especially for heat-vulnerable populations including people with chronic illness, young children, the elderly, and outdoor workers.

These risks compound when extremely hot days and nights occur in historically cooler climates where people are not accustomed to these events, in urban heat islands, and in neighborhoods with lower-income and more non-white residents that face disproportionate exposure to urban heat extremes across the U.S.

More warm nights also mean higher demand for air conditioning, which is less accessible to low-income households. Climate Central analysis found that 232 U.S. cities have experienced rising cooling demands from 1970 to 2022 due to rising temperatures. 

Higher cooling demand means higher energy bills and higher risks among households that lack or cannot afford to run air conditioning systems. Running air conditioners can further increase cooling demand both in the short term (as waste heat increases outdoor air temperatures) and in the long term (due to additional carbon pollution from running a rapidly-growing number of air conditioners globally). 

Why are nights warming so fast? 

Our warming climate makes heat extremes hotter, longer, and more frequent — especially at night.

Warming increases evaporation from the land surface, and a warmer atmosphere can hold more of that evaporated moisture, leading to higher humidity and more cloud cover. 

  • The resulting clouds can reduce daytime warming by reflecting some incoming heat from the sun back into space. 

  • But after the sun goes down, clouds can amplify nighttime warming by absorbing heat from the land surface and re-emitting that heat back down toward the ground. 

Parts of the globe that have experienced faster nighttime warming have also experienced increased cloud cover, specific humidity, and precipitation, likely due to the intensification of the water cycle in our warming climate. 


Did climate change influence hot days or nights today?

Climate Central’s Climate Shift Index system provides tools, data, custom maps, and local alerts to answer this question in real-time. Here are three ways to use the Climate Shift Index this summer:

  1. Use the tools. Climate Central’s Climate Shift Index map tool shows which parts of the world are experiencing high Climate Shift Index levels, every day. Explore the global Climate Shift Index map for today, tomorrow, and any day in the recent past.

  2. Create custom Climate Shift Index maps. The Climate Shift Index is now available in KML format. Fill out this form to join our pilot project, receive the KML links, and create custom Climate Shift Index maps. 

  3. Sign up for alerts. Sign up here to receive custom email alerts when strong Climate Shift Index levels are detected in your local area.

How is extreme heat affecting public health in your local area?

Check out HeatRisk, an interactive map tool from the National Weather Service now available for the contiguous U.S. This color-numeric index shows current and forecast risk of daily local heat-related impacts. The Centers for Disease Control and Prevention’s Heat and Health Tracker maps heat-related illnesses at the census tract level in real time. Use the National Integrated Heat Health Information System’s Heat Equity Mapper to explore the burden of extreme heat at the census tract level. Check the U.S. Department of Health and Human Services monthly Climate and Health Outlook for extreme heat outlooks in your region throughout the summer.  

What local measures are in place to protect people from heat?

The EPA maintains a Heat Island Community Actions Database of measures that communities are taking to mitigate local heat island effects. Reports from NOAA’s urban heat island mapping campaigns cover local risk reduction and adaptation strategies in 60+ U.S. cities. Track local climate-related hazards in real-time in the Climate Mapping for Resilience and Adaptation Portal, which also includes federal resources for long-term resilience planning. The NIHHIS provides heat preparedness and planning resources, including many resources in Spanish. 


Submit a request to SciLine from the American Association for the Advancement of Science or to the Climate Data Concierge from Columbia University. These free services rapidly connect journalists to relevant scientific experts. 

Browse maps of climate experts and services at regional NOAA, USDA, and Department of the Interior offices.  

Explore databases such as 500 Women Scientists, BIPOC Climate and Energy Justice PhDs, and Diverse Sources to find and amplify diverse expert voices. 

Reach out to your State Climate Office or the nearest Land-Grant University to connect with scientists, educators, and extension staff in your local area. 


Summer (June, July, August) daily temperature data (1970-2023) for 241 U.S. weather stations were obtained from the Applied Climate Information System. Following Wang et al. (2020), a compound hot summer day-night event is defined as the sequential occurrence of both a hot day and a hot night within a single 24-hour period during meteorological summer (June, July, August). Hot days and hot nights are defined as maximum and minimum daily temperatures, respectively, that are higher than their historical (1970-2023) 90th percentile for each specific calendar day during summer. Daily 90th percentiles were determined by ranking historical (1970-2023) data for each calendar day. 

We counted the annual occurrence of compound hot summer day-night events (defined above) at each station for every year from 1970 to 2023. Observed trends were summarized by comparing the average annual count of compound hot summer day-night events during the first decade (1970-1979) and last decade (2014-2023) of the period of analysis.

While Climate Matters local analyses typically include data from 247 stations, six were excluded from this analysis due to insufficient data: Dothan, Ala.; Hattiesburg, Miss.; Hazard, Ky; Jefferson City, Mo.; Twin Falls, Idaho; Wheeling, W. Va.